This invention relates to a resin composition comprised of a polyester capable of taking a crystal structure and also to a molded product using the resin composition. More particularly, the invention relates to a technique of facilitating crystallization of a biodegradable resin and ensuring satisfactory durability during practical use.
As a recent tendency toward environmental consciousness is increasing, attention has been paid to use of resin materials that are degradable in a natural environment and thus have so-called biodegradability.
Unlike hitherto known general-purpose resins, resins having biodegradability are prepared, for example, from nonfossil fuels and thus, have advantages: no problem is involved in shortage of resources; such resins contribute to resolution of a problem on waste treatment since they are degradable in a natural field; they are obtainable from natural resources such as sweet corn and the like; and the amount of CO2 gas that has been accepted as causing global warming can be suppressed. Hence, these resins are materials that are promised to attract further attention in future.
Among degradable resins, aliphatic polyesters, particularly polylactic acid, are high in melting point (170 to 180° C.), and are so excellent in material characteristics that the molded product obtained therefrom has good transparency, thus being expected as having wide utility.
It has been accepted that main applications of these biodegradable resins include, for example, materials for agriculture, forestry and fisheries (films, planting pots, fishing lines, fishnets and the like), civil engineering work materials (water retention sheet, nets for plants and the like), fields of packages and containers (those which are difficult to recycle owing to deposition of soil, food or the like), disposable goods such as convenience goods, sanitary goods, game products and the like. A further increase in future use thereof has been expected from the standpoint of environmental protection.
For instance, applications to electric and electronic articles such as television chassises, personal computer housings ad the like have been studied. When taking the applications to chassises and structural materials of such electric products into consideration, it is considered that a heat resistance of about 80° C. or over is necessary.
In recent years, in order to keep excellent characteristic properties for use as a material to be put into practice, importance has been placed on increase in degree of crystallization.
Especially, with respect to biodegradable polyesters, however, polylactic acid that is typical of the polyesters is a material which is poor in heat resistance and has a glass transition temperature (Tg) of approximately 60° C. When the molded product obtained therefrom is exposed to a temperature exceeding the glass transition temperature, it is softened and deformed and thus, has a problem in practical use.
It will be noted that a heat resistance in practical use means to ensure a stiffness (modulus of elasticity) of about 100 MPa in the vicinity of 80° C.
In order to increase the heat resistance of a biodegradable polyester, mention is made, for example, of a method for adding an inorganic filler having a heat resistance, such as talc, mica or the like. By this, the mechanical properties and the hardness of material can be improved.
However, only addition of inorganic filler to the resin involves a difficulty in ensuring a satisfactory heat resistance in practice.
In the related art, there has been proposed a technique of improving a heat resistance of polylactic acid through thermal treatment during or after molding.
Polylactic acid is a kind of polyester that is able to take a crystal structure, but is a polymer that is unlikely to crystallize. Hence, when polylactic acid is molded according to a method as used in ordinary general-purpose resins. The resulting product becomes amorphous in nature and is eventually poor in mechanical strength and susceptible to thermal deformation.
Nevertheless, when a thermal treatment is performed on the product during or after molding to facilitate crystallization, the heat resistance thereof is improved.
However, the crystallization with a thermal treatment takes a long time and thus, has a problem in a manufacturing practice.
For example, in case where general-purpose resins are used, an injection molding procedure is ordinarily carried out by an about one minute molding cycle. In this connection, it has been considered that for thermally treating a molded product using polylactic acid in such a way that the product is crystallized to such an extent as to ensure mechanical strength sufficient for practical use, too much time is taken in practice.
Where no nucleating agent is added in the crystallization step, a frequency of natural occurrence of crystal nuclei is so low that the crystal size becomes on the order of micrometers. This presents a problem in that a finally obtained molded product becomes clouded and is thus poor in transparency, thus placing limitation on a range of practical use.
As to polyesters capable of taking a crystal structure, one of measures for solving such problems as set out hereinabove and also for facilitating crystallization of the resin is addition of a nucleating agent.
The nucleating agent is one, which becomes primary crystal nuclei and promotes the growth of crystals of a crystalline polymer. Broadly, substances that promote crystallization of a crystalline polymer, or are able to improve a crystallization speed of polymer may be called nucleating agent.
If a nucleating agent is added to a polymer, crystals become more finely thereby obtaining an improved effect of stiffness on a finally obtained resin and an improved effect of transparency.
Since the crystallization speed is improved during the curse of molding, an advantage is obtained in shortening a time required for this step.
Such effects as stated above have been actually confirmed with respect to other types of crystalline polymers.
For instance, polypropylene (which may be hereinafter referred sometimes to as PP) is improved in stiffness and transparency when a nucleating agent is added.
In this instance, a sorbitol substance is applied, for example, as a nucleating agent and it is considered that a three-dimensional network structure of the substance effectively acts on the improvement.
Besides, for a material of a metal salt type, mention is made, for example, aluminium hydroxy-di(t-butylbenzoate), sodium (4-t-butylphenyl)phosphate, sodium methylenebis(2,4-di-t-butylphenyl)phosphate and the like.
In this connection, however, where a polyester such as polylactic acid is used as a resin, a problem in practical applications is involved with respect to a nucleating agent to be applied.
For instance, where talc is used, its amount should be on the order of several tens of % in order to ensure a satisfactory nucleating effect. However, such an amount becomes too high in content of talc in a resin, with the attendant problem that a finally obtained resin composition is not ensured to obtain mechanical strength thereof that is satisfactory in practice.
The high content of talc in resin causes cloudness to occur and transparency to be degraded, thus leading to a problem in that a range of practical use is narrowly limited.
In the related art, a technique of using sorbitol substances as a nucleating agent has been disclosed (see, for example, Japanese Patent Laid-open No. Hei 10-158369) for application to aliphatic polyesters. It is also disclosed that the substance has an appreciable crystallization effect on polylactic acid.
For promoting crystallization by addition of other types of nucleating agents, a technique of adding at least one member selected from compounds having a melting point of 40 to 300° C. and consisting of aliphatic carboxylic acid amides, aliphatic carboxylic acid salts, aliphatic alcohols and aliphatic carboxylic acid esters has been proposed, for example, as a transparent nucleating agent to be added to aliphatic polyesters (see, for example, Japanese Patent Laid-open No. Hei 9-278991).
Further, there have been proposed techniques including a technique wherein at least one organic compound selected, as a transparent nucleating agent, from those organic compounds having a melting temperature or softening point of 80 to 300° C. and a melting entropy of 10 to 100 cal/K/mol (see, for example, Japanese Patent Laid-open No. Hei 11-5849) and a technique wherein an aliphatic acid ester having a specific type of structure is added, as a clarifying agent, to a polylactic acid resin (see, for example, Japanese Patent Laid-open No. Hei 11-116783).
Especially, with respect to polylactic acid, there have been proposed techniques of providing resin compositions, which are excellent in heat resistance and impact strength, by formulating a certain type of heterocyclic compound in polylactic acid (see Japanese Patent Laid-open Nos. 2004-352872 and 2004-352873).
Phthalic acid hydrazide is exemplified as the heterocyclic compound. In an example wherein this heterocyclic compound and talc are added in combination, crystallinity of polylactic acid is stated as being improved.
In industrial products prepared by use of polyesters, it has been accepted that to ensure durability relative to hydrolysis is important from the standpoint of practical use.
The degree of hydrolysis differs depending on the type and use environment of polyester to be applied. Taking the use period required for individual molded products into account, hydrolysis does not always present a problem involved in practice. Nevertheless, especially, in case where a biodegradable polyester is used, it will become important to ensure service durability against hydrolysis.
More particularly, with a short use period (or with a short time), rapid decomposition is preferred. In this connection, however, with a long use period (or with a long time), hydrolysis has to be suppressed.
For instance, with the case of applications to chassises of electric products, electronic appliances and the like, long-term reliability of about several to ten years is required and thus, mechanical properties such as tensile strength, bending strength, impact resistance and the like have to be maintained at a satisfactory level from a practical standpoint over such a period as mentioned above.
With respect to a technique of improving long-term reliability of degradable polyesters, many proposals have been hitherto made. However, to ensure material reliability simultaneously with an improvement in crystallinity of such a resin as set out hereinabove has never been attained satisfactorily.